enhance(#3911): hooks declare their crash policy (#3960)

* enhance(#3911): give hooks an exit seam that needs no build

ADR-3889 Phase 7 foundation. The 19 shipped enforcement hooks hold 91 of the
epic's 128 terminators and cannot reach `terminateNow` today.

The obvious route — requiring `gsd-core/bin/lib/cli-exit.cjs`, as
gsd-agent-isolation-guard.js already does for two other modules — is rejected.
That precedent carries its own warning (#3582): those files are tsc output,
gitignored and absent on a raw plugin-marketplace or git-clone install, so the
hook must first call ensureRuntimeBuild() to self-heal. Making the module a
hook needs IN ORDER TO TERMINATE depend on a build inverts the dependency, and
its failure mode is precisely the fail-open this phase exists to remove: a
guard that cannot terminate cannot deny. `lint-hooks-runtime-build-seam`
already encodes that concern, and Design B would have had to add an
ensureRuntimeBuild() call to all 19 hooks to satisfy it.

So `hooks/lib/` becomes a third emit location for cli-exit and a fifth for the
registry, preserving the invariant `src/cli-exit.cts`'s own header states: it
imports nothing but node:fs and its sibling registry, and the generator
dual-emits that sibling alongside each copy so a relative require resolves next
to whichever copy loaded it. Shipping needed no change — build-hooks.js already
declares HOOKS_SUBDIRS_TO_COPY = ['lib'].

Proven, not asserted: the two files are copied into an otherwise-empty tmpdir
and a child process requires them and terminates — PASS exits 0, HOOK_DENY
exits 2 with the payload on both stdout and stderr. That test fails the moment
the hooks copy gains a require reaching outside hooks/lib/.

Also fixed inline: the registry's fifth target let any `--write` test overwrite
the real committed hooks/lib/exit-code-registry.js, because the test helper
derived only three of the other output paths. It now redirects all five, and a
regression test asserts every committed artifact is byte-identical after a
redirected write.

Install-tree goldens pick up the two new shipped paths across 11 runtimes —
insertions only, no removals. lint:ci was green while they were stale, so this
was found by regenerating rather than by a gate.

Verification runs on the remote runner.

Refs #3911

* enhance(#3911): declare a crash policy, and migrate the write guard

Adds `hooks/lib/hook-exit.js` — the hook-facing vocabulary over `terminateNow`,
hand-written because the cli-exit copy beside it is generated:

  allow(payload)          exit 0
  deny(payload, stderr?)  exit 2
  crash(onCrash, payload) whichever the hook DECLARED

`crash()` takes the policy as a required argument with no default, which is the
whole mechanism: fail-open by accident stops being expressible. A hook must
name ALLOW or DENY at the call site, and an unrecognized value terminates
INTERNAL rather than guessing. Fail-open stays legal; fail-open by omission
does not.

`gsd-write-guard.js` is the first hook migrated, all 12 sites, and it exposed a
gap in the seam. `terminateNow`'s doc comment justified its fd-2 write by
citing this hook's `emitBlock` — but modeled it as sending the same bytes to
both streams, when `emitBlock` actually sends full JSON to stdout and only the
bare `reason` string to stderr, because Kimi's hook bus feeds stderr verbatim
back to the model. Migrating as written would have turned a readable sentence
into a JSON blob for Kimi-backed agents.

#3911 requires both "all 19 hooks terminate through terminateNow" and "no
hook's effective default changes". Those are jointly satisfiable only by
teaching the seam to carry a distinct stderr payload, so `terminateNow` gains
an optional third argument: omitted, behavior is byte-for-byte what it was; a
string is written raw, which is exactly the Kimi case. The doc comment's
inaccurate claim about emitBlock is corrected in place.

Proven rather than asserted: the pre-migration file is reconstructed from HEAD
and driven with the same catastrophic-shrink payload as the migrated one —
exit code, stdout and stderr all byte-identical.

Verification runs on the remote runner.

Refs #3911

* enhance(#3911): all 19 hooks terminate through the seam

Migrates the remaining 18 enforcement hooks onto allow/deny/crash. An AST walk
now reports zero `process.exit(` call sites across every `hooks/*.js` — down
from the 91 the census measured.

Each hook with an outer catch declares its policy once, at module top, with the
reason that policy is right for that specific guard: a read guard that cannot
scan must not block the read; a statusline that renders every prompt must
degrade rather than crash; an injection scanner must not retroactively block a
result already returned. Those sentences are the deliverable — they are what
turns fail-open-by-accident into fail-open-on-purpose. No hook's effective
default changed.

Wiring exposed two defects, both fixed here rather than noted.

A SECOND stdout/stderr-splitting site turned up in `gsd-workflow-guard.js`'s
`emitForceAddBlock`, matching the pattern already known from the write guard —
full JSON to stdout, bare reason to stderr for the Kimi bus. It uses the
`stderrPayload` argument added in the previous commit, which is now carrying
its second real caller rather than one special case.

More seriously, `terminateNow` emitted both streams inside ONE try, so a
payload that failed to serialize aborted before the stderr write ever ran. The
two windsurf guards write nothing to stdout on a block and only a reason string
to stderr, so `deny(undefined, reason)` exited 2 with EMPTY stderr — a deny
that silently loses its reason, which is the exact "fails with success" class
this epic exists to close. The streams are now emitted independently, each with
its own guard, and `undefined` means "nothing to write for this stream" rather
than an error. Regression tests inject a throwing write on one fd and assert
the other still receives its payload; they fail against the single-try version.

Byte-identity was proven per hook, not assumed: each pre-change file is
reconstructed from HEAD and driven side by side with the migrated one across
its normal path, its deny path, malformed stdin and empty stdin — exit code,
stdout and stderr compared.

Verification runs on the remote runner.

Refs #3911

* enhance(#3911): harden the three shell hooks, and pin every hook's policy

`gsd-phase-boundary.sh`, `gsd-session-state.sh` and `gsd-validate-commit.sh`
gain `set -euo pipefail`.

The expected hazard did not materialize, and that is worth recording: every
intentionally-non-zero command in all three is already the condition of an
`if`/`elif`, which `set -e` never fires on, and none of them reads a
possibly-unset variable or pipes through a grep that may legitimately match
nothing. No `|| true` guards were needed. Each hook was still checked
command-by-command before the flags went in rather than after.

Twenty-one before/after cases across the three hooks — disabled and enabled,
planning and non-planning, missing STATE.md, malformed JSON, the Kimi payload
shape, quoted and unquoted `-m`, valid and over-long Conventional Commits —
all match on exit code, stdout and stderr.

The hardening is shown to actually fire, not merely added: with a stubbed
`node` that fails at the JSON-emit step, phase-boundary and session-state go
from silently exiting 0 with empty stdout to failing visibly with the error
surfaced. No such case could be constructed for `gsd-validate-commit.sh`,
whose every statement already sits inside an if-condition — recorded as
unproven rather than claimed.

`tests/hooks-crash-policy.test.cjs` adds the per-hook coverage the issue asks
for, table-driven over all 19 hooks rather than 76 hand-written cases: normal
allow, deny where a deny path exists, crash-honors-the-declared-policy, and an
unclosed-stdin case — the one `process.exitCode` structurally cannot serve. The
deny assertions encode each hook's ACTUAL stream split rather than a uniform
shape, since four of the six deliberately differ. A drift guard enumerates
`hooks/*.js` and fails if a terminating hook is ever added without a row.

Writing those tests surfaced two hooks that emit a block decision in their JSON
body and exit 0. Both were checked rather than assumed, and neither is a
fails-with-success: `gsd-read-injection-scanner.js` is PostToolUse, where the
tool has already run and exit 2 has no meaning, and `gsd-cursor-subagent-start.js`
follows Cursor's JSON-body protocol. They are deliberately left alone — a
mechanical sweep to `deny()` would have broken exactly these two.

Verification runs on the remote runner.

Refs #3911

* fix(#3838): the commit validator says when it could not validate

#3911 claims to subsume #3838. Measurement said otherwise, so this closes it
for real rather than by assertion.

`set -euo pipefail`, added earlier on this branch, does NOT fix #3838: bash
exempts a command used as an `if` condition from `set -e`, and all three of the
hook's swallow-and-pass sites are exactly that shape. Verified against the
hardened hook with a node shim that fails only the classifier call — a
non-conforming commit still exited 0 with empty stdout AND empty stderr,
indistinguishable from "your commit conforms". That is the defect verbatim.

All three sites named in #3838 now capture the real exit status instead of
consuming it as a condition, and each distinguishes its genuine negative from
"could not run":

- the classifier: 0 = is a git commit, 1 = genuinely not one, anything else =
  could not classify. Its `node -e` now wraps the require and the call in
  try/catch and exits 3 on a throw, so a broken require chain can never be
  mistaken for `isGitSubcommand` legitimately returning false — which is the
  arm that matters, since `token-scanner.cjs` is a gitignored build artifact
  and a fresh checkout lands there.
- the opt-in config read and the JSON command extraction get the same
  treatment.

On "could not run" the hook emits a diagnostic to stderr naming which check
failed and why, then exits 0. The issue confirms this is safe — it is a
PreToolUse hook, so stderr does not disturb the JSON protocol — and ranks it
the smallest sufficient fix. The gate still fails open, but it can no longer do
so silently, which is the whole complaint: a validator that disables itself
quietly costs more than one that is absent, because it is trusted.

Both controls are unchanged and pinned by tests: a conforming commit still
passes silently, a non-conforming one still exits 2 with its existing block
payload. The defect test asserts stderr is non-empty and names the failure; it
fails against the pre-fix hook.

Verification runs on the remote runner.

Refs #3911, #3838

* docs(#3911): document the hook crash-policy contract

Reference and Explanation via a new docs/features fragment (FEATURES.md is
generated from it), INVENTORY rows for the three new hooks/lib files, and an
ARCHITECTURE note on the hooks section.

How-To: docs/how-to/declare-a-hook-crash-policy.md, indexed from docs/README.md
— a hook author now has to choose and declare a crash policy, which is more
than one step and crosses into which harness protocol their hook speaks. It
covers allow/deny/crash, writing an ON_CRASH reason that is actually useful,
when a deny needs a distinct stderr payload, the two hooks whose harness reads
a JSON-body decision and must NOT use deny(), and what to do when a check
cannot run at all — with #3838 as the worked example.

Refs #3911

* test(#3911): prove the seam actually ships, and stop hand-rolling temp cleanup

Two review findings.

The acceptance criterion 'hooks/dist/** stays in parity via the build seam
(lint:hooks-runtime-build-seam)' was misstated and unmet: that lint checks
something else — that a hook requiring a compiled gsd-core/bin/lib module also
calls ensureRuntimeBuild(). Nothing exercised that the three new hooks/lib
files reach hooks/dist/lib at all. That gap is not theoretical: #770 is a
recorded ship-blocking bug where a new hook never shipped because a copy list
missed it. The suite now builds dist through the repo's own ensureBuiltHooks(),
byte-compares each shipped copy against its source, and spawns a child that
requires the SHIPPED dist copy and denies — which is what catches a copy that
exists but cannot resolve its sibling registry.

gsd-validate-commit.sh hand-duplicated mktemp/run/rm three times; one idempotent
trap on EXIT replaces them, guarded so cleanup cannot alter the exit status.
Behavior-neutral across five cases, with temp-file counts taken before and
after each run.

Refs #3911

* fix(#3911): stage transitive hook lib requires, not just one level

The remote run returned 7 failures across 3 real causes.

The important one is a PRODUCTION bug this phase exposed rather than caused.
`writeCursorHooksJson` scanned each hook script for `./lib/X` requires exactly
one level deep and never re-scanned the lib files it staged for their own
sibling requires. Nothing had a transitive lib dependency before, so the gap
was invisible. Adding hook-exit.js -> cli-exit.js -> exit-code-registry.js
made real Cursor installs ship a bundle that dies at require time with
MODULE_NOT_FOUND. It now walks to a fixed point, and a real installed Cursor
hook runs to completion.

The staging harness in shared-hooks-dir-resolution hand-copied its fixture, so
the injection scanner crashed at require time and its exit-1 was being read as
a policy decision. Migrated to copyScriptWithDeps, which walks the require
graph — the repo's recorded rule for this class, since adding another
copyFileSync keeps it alive for the next person.

The missing-lib-source test in cursor-hook-workspace-roots hardcoded which lib
file it expected to be named in the abort message; the same throw now fires for
a different file first. Its assertion is unchanged in substance — staging still
must abort rather than ship a broken hook — only the name is no longer pinned.

The last one was my own test asserting an uppercase reason code. Measured
against origin/next: the pre-change hook emits the same lowercase
'config_unreadable', so the test was wrong, not the migration. Corrected to the
real value rather than making the code match the test.

Verification runs on the remote runner.

Refs #3911

* chore(#3911): regenerate the cursor install-tree golden

The staging fix means a Cursor install now correctly carries the two
transitive lib files it was silently missing. Additive only — no path was
removed. The golden diff is the evidence the packaging defect was real.

Refs #3911

* chore(#3911): backfill the changeset PR number

Refs #3911

* fix(#3911): a git probe that timed out is not a negative

A macOS CI lane failed three deny cases at 2084ms, 2112ms and 2177ms — just
past the 2000ms budget these hooks give their git probes. The three that passed
took 72ms, 595ms and 651ms. Under shard contention `git rev-parse` overruns,
the hook reads the non-zero result as "not a git repo", and allows with exit 0
and empty stdout AND empty stderr. Under load, the guards silently stop
guarding. That is ADR-3889's thesis exactly, sitting inside the security hooks
this phase is about.

The repo had already recognized the class in one place — gsd-cursor-subagent-start.js
fail-closed-denies on `git_timed_out` (#3045) — but nowhere else.

`hooks/lib/git-probe.js` classifies a probe's outcome, distinguishing a real
non-zero exit from ETIMEDOUT, a signal kill, and a spawn failure, rather than
folding all four into `status !== 0`. Three guards route their eight git probes
through it.

The resolution is the same shape #3838 took, and the same one that issue
endorsed as smallest-sufficient: fail open, but loudly. **No exit code changes
on any path** — a developer on a loaded machine is still not blocked, which
keeps #3911's declaration-pass contract intact for exit codes. What changes is
that the hook now says on stderr which probe could not answer, instead of
presenting silence as a clean verdict.

Scope was checked across every hooks/*.js, not just the three that failed:
gsd-agent-isolation-guard spawns no git; gsd-statusline's two probes gate only
a cosmetic display segment, not an allow/deny decision, and are left alone.

The C2 deny assertion was a real-race test — it demanded exit 2 while a slow
git legitimately yields 0. It now requires the hook to either deny, or allow
with a diagnostic naming the probe that could not run; a silent allow still
fails, so the assertion is not vacuous. A deterministic regression stubs git on
PATH to sleep past the budget rather than waiting for load to reproduce it.

Verification runs on the remote runner.

Refs #3911

* test(#3911): a PATH shim cannot intercept the hooks' git spawn on Windows

The deterministic timeout regression stubbed git on PATH and asserted the
guard reports rather than silently allows. It passes on Linux and macOS and
failed on Windows in 83ms and 176ms — the stub was never invoked at all.

Mechanism: the hooks call spawnSync('git', args) with no shell:true, so on
Windows CreateProcess resolves git.exe only and never a PATH .cmd shim. The
git.cmd branch could not have worked and is removed rather than left implying
a Windows path that does. Adding shell:true to the hooks to serve a test would
change product behavior and widen an injection surface, so the case is skipped
on win32 only, with the mechanism written into the skip reason so a future
reader does not 'fix' it that way.

Linux and macOS keep the coverage, and macOS is where the underlying fail-open
was actually caught.

Refs #3911

---------

Co-authored-by: sim <sim@local>

tests/cli-exit.test.cjs

@@ -21,6 +21,13 @@ const IO_PATH = path.resolve(__dirname, '../gsd-core/bin/lib/io.cjs');
 const SCRIPTS_CLI_EXIT_PATH = path.resolve(__dirname, '../scripts/lib/cli-exit.cjs');
 const EXIT_CODE_REGISTRY_PATH = path.resolve(__dirname, '../gsd-core/bin/lib/exit-code-registry.cjs');
 
+// #3911 (ADR-3889 Phase 7): the THIRD emitted copy, for hooks/ consumers that
+// must terminate through terminateNow without depending on any build
+// artifact (gsd-core/bin/lib is gitignored tsc output, absent on a raw
+// plugin-marketplace or git-clone install).
+const HOOKS_CLI_EXIT_PATH = path.resolve(__dirname, '../hooks/lib/cli-exit.js');
+const HOOKS_EXIT_CODE_REGISTRY_PATH = path.resolve(__dirname, '../hooks/lib/exit-code-registry.js');
+
 const { EXIT_CODES } = require(EXIT_CODE_REGISTRY_PATH);
 const REGISTERED_NAMES = EXIT_CODES.map((e) => e.name);
 const VERSIONS = ['v1', 'v2'];
@@ -1147,6 +1154,126 @@ describe('#3906: terminateNow', () => {
     assert.equal(r.stderr, '');
   });
 
+  test('#3911: HOOK_DENY with NO stderrPayload arg — backward-compatible default: fd1 and fd2 both get the serialized payload', () => {
+    const r = spawnTerminateNow([
+      `const c = require(${JSON.stringify(BUILT_CLI_EXIT_PATH)});`,
+      `c.terminateNow('HOOK_DENY', { reason: 'blocked-default' });`,
+    ]);
+    assert.equal(r.status, 2);
+    assert.deepEqual(JSON.parse(r.stdout), { reason: 'blocked-default' });
+    assert.deepEqual(JSON.parse(r.stderr), { reason: 'blocked-default' });
+  });
+
+  test('#3911: HOOK_DENY with a STRING stderrPayload — fd1 gets JSON, fd2 gets the raw string verbatim', () => {
+    const r = spawnTerminateNow([
+      `const c = require(${JSON.stringify(BUILT_CLI_EXIT_PATH)});`,
+      `c.terminateNow('HOOK_DENY', { decision: 'block', reason: 'shrink too large' }, 'shrink too large');`,
+    ]);
+    assert.equal(r.status, 2);
+    assert.deepEqual(JSON.parse(r.stdout), { decision: 'block', reason: 'shrink too large' });
+    assert.equal(r.stderr, 'shrink too large', `expected raw string on stderr, got: ${r.stderr}`);
+    assert.throws(() => JSON.parse(r.stderr), SyntaxError,
+      'a raw reason string must NOT itself be JSON-parseable as an object');
+  });
+
+  test('#3911: HOOK_DENY with an OBJECT stderrPayload — fd2 gets that object serialized, not the fd1 payload', () => {
+    const r = spawnTerminateNow([
+      `const c = require(${JSON.stringify(BUILT_CLI_EXIT_PATH)});`,
+      `c.terminateNow('HOOK_DENY', { full: 'stdout-payload' }, { distinct: 'stderr-payload' });`,
+    ]);
+    assert.equal(r.status, 2);
+    assert.deepEqual(JSON.parse(r.stdout), { full: 'stdout-payload' });
+    assert.deepEqual(JSON.parse(r.stderr), { distinct: 'stderr-payload' });
+  });
+
+  test('#3911: a PASS outcome with a stderrPayload writes NOTHING to stderr — stderr is a deny-only channel', () => {
+    const r = spawnTerminateNow([
+      `const c = require(${JSON.stringify(BUILT_CLI_EXIT_PATH)});`,
+      `c.terminateNow('PASS', { ok: true }, 'should never reach stderr');`,
+    ]);
+    assert.equal(r.status, 0);
+    assert.deepEqual(JSON.parse(r.stdout), { ok: true });
+    assert.equal(r.stderr, '', `expected empty stderr for a non-deny outcome; got: ${r.stderr}`);
+  });
+
+  test('#3911: terminateNow(PASS, undefined) exits 0 with empty stdout', () => {
+    const r = spawnTerminateNow([
+      `const c = require(${JSON.stringify(BUILT_CLI_EXIT_PATH)});`,
+      `c.terminateNow('PASS', undefined);`,
+    ]);
+    assert.equal(r.status, 0, `stderr: ${r.stderr}`);
+    assert.equal(r.stdout, '', `expected empty stdout, got: ${r.stdout}`);
+    assert.equal(r.stderr, '', `expected empty stderr, got: ${r.stderr}`);
+  });
+
+  test('#3911: terminateNow(HOOK_DENY, undefined) exits 2 with EMPTY stdout and EMPTY stderr', () => {
+    const r = spawnTerminateNow([
+      `const c = require(${JSON.stringify(BUILT_CLI_EXIT_PATH)});`,
+      `c.terminateNow('HOOK_DENY', undefined);`,
+    ]);
+    assert.equal(r.status, 2, `stderr: ${r.stderr}`);
+    assert.equal(r.stdout, '', `expected empty stdout, got: ${r.stdout}`);
+    assert.equal(r.stderr, '', `expected empty stderr, got: ${r.stderr}`);
+  });
+
+  // The defect this whole file is regressing (#3911): `deny(undefined,
+  // 'some reason')` used to exit 2 with EMPTY stderr because the fd1 write of
+  // `undefined` threw (JSON.stringify(undefined) -> undefined,
+  // Buffer.from(undefined,'utf8') throws) and the SHARED try/catch aborted
+  // before the fd2 write of `stderrPayload` ever ran.
+  test('#3911: terminateNow(HOOK_DENY, undefined, "reason text") exits 2 with EMPTY stdout and stderr EXACTLY "reason text"', () => {
+    const r = spawnTerminateNow([
+      `const c = require(${JSON.stringify(BUILT_CLI_EXIT_PATH)});`,
+      `c.terminateNow('HOOK_DENY', undefined, 'reason text');`,
+    ]);
+    assert.equal(r.status, 2, `stderr: ${r.stderr}`);
+    assert.equal(r.stdout, '', `expected empty stdout, got: ${r.stdout}`);
+    assert.equal(r.stderr, 'reason text', `expected exactly "reason text" on stderr, got: ${r.stderr}`);
+  });
+
+  // ── #3911 regression: the two stream emissions are INDEPENDENT ───────────
+  // These two tests are the direct regression coverage for the defect: a
+  // shared try/catch around both writes meant a failure serializing/writing
+  // fd 1 aborted before fd 2 (or vice versa) ever ran. Both must FAIL against
+  // the pre-fix single-try-block implementation.
+  test('#3911 regression: fd 1 write throws but fd 2 STILL receives its payload, exit code unchanged', () => {
+    const r = spawnTerminateNow([
+      `const fs = require('node:fs');`,
+      `const origWriteSync = fs.writeSync;`,
+      `fs.writeSync = (fd, ...rest) => {`,
+      `  if (fd === 1) throw new Error('injected fd1 failure');`,
+      `  return origWriteSync(fd, ...rest);`,
+      `};`,
+      `const c = require(${JSON.stringify(BUILT_CLI_EXIT_PATH)});`,
+      `c.terminateNow('HOOK_DENY', { reason: 'fd1-throws' });`,
+    ]);
+    assert.equal(r.status, 2, `exit code must be unchanged by the fd1 failure; stderr: ${r.stderr}`);
+    assert.equal(r.stdout, '', 'fd1 write failed, so stdout must be empty (not a partial payload)');
+    assert.deepEqual(
+      JSON.parse(r.stderr), { reason: 'fd1-throws' },
+      `fd2 must still receive its payload despite the fd1 failure; got: ${r.stderr}`,
+    );
+  });
+
+  test('#3911 regression: fd 2 write throws but fd 1 STILL receives its payload, exit code unchanged', () => {
+    const r = spawnTerminateNow([
+      `const fs = require('node:fs');`,
+      `const origWriteSync = fs.writeSync;`,
+      `fs.writeSync = (fd, ...rest) => {`,
+      `  if (fd === 2) throw new Error('injected fd2 failure');`,
+      `  return origWriteSync(fd, ...rest);`,
+      `};`,
+      `const c = require(${JSON.stringify(BUILT_CLI_EXIT_PATH)});`,
+      `c.terminateNow('HOOK_DENY', { reason: 'fd2-throws' });`,
+    ]);
+    assert.equal(r.status, 2, `exit code must be unchanged by the fd2 failure; stderr: ${r.stderr}`);
+    assert.deepEqual(
+      JSON.parse(r.stdout), { reason: 'fd2-throws' },
+      `fd1 must still receive its payload despite the fd2 failure; got: ${r.stdout}`,
+    );
+    assert.equal(r.stderr, '', 'fd2 write failed, so stderr must be empty (not a partial payload)');
+  });
+
   // Cross-platform IO-failure injection: a monkeypatched THROWING fs.writeSync,
   // restored implicitly by process exit — never chmod (CONTRIBUTING.md /
   // CLAUDE.md: mode-bit tricks are bypassed by root/CI and leak resources).
@@ -1366,6 +1493,61 @@ describe('#3906: cross-copy — the built copy and the scripts copy agree', () =
     }
   });
 
+  // #3911 (ADR-3889 Phase 7) A6: EXTENDS the two-copy parity above to the
+  // THIRD emitted copy (hooks/lib/cli-exit.js + hooks/lib/exit-code-registry.js)
+  // rather than duplicating it. Outcome names are enumerated from the hooks
+  // registry itself — never hardcoded — so a future registry addition is
+  // covered automatically and a hooks-registry omission fails loudly here
+  // instead of silently under-testing the hooks copy.
+  test('the hooks copy agrees with both existing copies for every registered outcome/version', () => {
+    const built = require(BUILT_CLI_EXIT_PATH);
+    const scripts = require(SCRIPTS_CLI_EXIT_PATH);
+    const hooks = require(HOOKS_CLI_EXIT_PATH);
+    const { EXIT_CODES: hooksExitCodes } = require(HOOKS_EXIT_CODE_REGISTRY_PATH);
+    const hooksNames = hooksExitCodes.map((e) => e.name);
+
+    assert.deepStrictEqual(
+      [...hooksNames].sort(), [...REGISTERED_NAMES].sort(),
+      'the hooks registry must declare the exact same outcome set as the primary registry',
+    );
+
+    for (const version of VERSIONS) {
+      for (const outcome of ['PASS', 'FAIL', ...REGISTERED_NAMES]) {
+        const fromBuilt = built.projectOutcome(outcome, version);
+        const fromScripts = scripts.projectOutcome(outcome, version);
+        const fromHooks = hooks.projectOutcome(outcome, version);
+        assert.equal(fromScripts, fromBuilt, `scripts vs built disagree for ${outcome}/${version}`);
+        assert.equal(fromHooks, fromBuilt, `hooks vs built disagree for ${outcome}/${version}`);
+      }
+    }
+  });
+
+  // #3911 A6: the json-error-mode cell is a `globalThis`-backed shared cell
+  // (see the #3906 "ONE json-error-mode cell" tests above for the built/scripts
+  // pair) — prove the hooks copy reads and writes the SAME cell, not a third,
+  // independent module-level flag that would silently diverge.
+  test('the json-error-mode cell is genuinely shared across all three copies (built, scripts, hooks)', () => {
+    const built = require(BUILT_CLI_EXIT_PATH);
+    const scripts = require(SCRIPTS_CLI_EXIT_PATH);
+    const hooks = require(HOOKS_CLI_EXIT_PATH);
+    const saved = built.getJsonErrorMode();
+    try {
+      built.setJsonErrorMode(true);
+      assert.equal(scripts.getJsonErrorMode(), true, 'scripts must observe the mode set through built');
+      assert.equal(hooks.getJsonErrorMode(), true, 'hooks must observe the mode set through built');
+
+      hooks.setJsonErrorMode(false);
+      assert.equal(built.getJsonErrorMode(), false, 'built must observe the mode set through hooks');
+      assert.equal(scripts.getJsonErrorMode(), false, 'scripts must observe the mode set through hooks');
+
+      scripts.setJsonErrorMode(true);
+      assert.equal(built.getJsonErrorMode(), true, 'built must observe the mode set through scripts');
+      assert.equal(hooks.getJsonErrorMode(), true, 'hooks must observe the mode set through scripts');
+    } finally {
+      built.setJsonErrorMode(saved);
+    }
+  });
+
   test('scripts/lib/cli-exit.cjs + its sibling exit-code-registry.cjs load standalone, no gsd-core sibling', (t) => {
     const dir = createTempDir('gsd-3906-standalone-');
     t.after(() => cleanup(dir));
@@ -1414,3 +1596,168 @@ describe('#3906: cross-copy — the built copy and the scripts copy agree', () =
     );
   });
 });
+
+// ─── #3911 (ADR-3889 Phase 7, issue #3911): the hooks copy is load-bearing ──
+//
+// The WHOLE POINT of hooks/lib/cli-exit.js is that a shipped enforcement hook
+// can terminate through terminateNow WITHOUT depending on any build
+// artifact. gsd-core/bin/lib is gitignored tsc output and is ABSENT on a raw
+// plugin-marketplace or git-clone install. These tests prove that property
+// hermetically: by copying ONLY hooks/lib/cli-exit.js and its sibling
+// hooks/lib/exit-code-registry.js into a fresh, otherwise-empty tmpdir (no
+// gsd-core sibling, no node_modules) and requiring the copy from a CHILD
+// process rooted there.
+describe('#3911: hooks/lib/cli-exit.js loads and terminates with no build present (A4, load-bearing)', () => {
+  /** Copy both hooks/lib artifacts into a fresh, otherwise-empty tmpdir. */
+  function makeStandaloneHooksCopy(t) {
+    const dir = createTempDir('gsd-3911-hooks-standalone-');
+    t.after(() => cleanup(dir));
+    const copiedExit = path.join(dir, 'cli-exit.js');
+    const copiedRegistry = path.join(dir, 'exit-code-registry.js');
+    fs.copyFileSync(HOOKS_CLI_EXIT_PATH, copiedExit);
+    fs.copyFileSync(HOOKS_EXIT_CODE_REGISTRY_PATH, copiedRegistry);
+    return { dir, copiedExit, copiedRegistry };
+  }
+
+  // This test must FAIL if hooks/lib/cli-exit.js ever gains a require
+  // reaching outside hooks/lib/ (e.g. back into gsd-core/bin/lib, or a
+  // node_modules package): the tmpdir contains NOTHING else, so any such
+  // require resolves to MODULE_NOT_FOUND and the child crashes before
+  // terminateNow ever runs, failing every assertion below.
+  test('terminateNow(PASS) exits 0 with the payload on stdout, from a build-free tmpdir', (t) => {
+    const { dir, copiedExit } = makeStandaloneHooksCopy(t);
+    const r = toLegacyResult(runNode(['-e', [
+      `const c = require(${JSON.stringify(copiedExit)});`,
+      `c.terminateNow('PASS', { ok: true, from: 'hooks-standalone' });`,
+    ].join('\n')], { cwd: dir, timeoutMs: PROBE_TIMEOUT_MS }));
+
+    assert.ok(
+      !r.stderr.includes('MODULE_NOT_FOUND'),
+      `the hooks copy must not require anything outside hooks/lib/; got: ${r.stderr.slice(0, 400)}`,
+    );
+    assert.equal(r.status, 0, `stderr: ${r.stderr}`);
+    assert.deepEqual(JSON.parse(r.stdout), { ok: true, from: 'hooks-standalone' });
+  });
+
+  test('terminateNow(HOOK_DENY) exits 2 with the payload on BOTH stdout and stderr, from a build-free tmpdir', (t) => {
+    const { dir, copiedExit } = makeStandaloneHooksCopy(t);
+    const r = toLegacyResult(runNode(['-e', [
+      `const c = require(${JSON.stringify(copiedExit)});`,
+      `c.terminateNow('HOOK_DENY', { reason: 'blocked-by-guard', from: 'hooks-standalone' });`,
+    ].join('\n')], { cwd: dir, timeoutMs: PROBE_TIMEOUT_MS }));
+
+    assert.ok(
+      !r.stderr.includes('MODULE_NOT_FOUND'),
+      `the hooks copy must not require anything outside hooks/lib/; got: ${r.stderr.slice(0, 400)}`,
+    );
+    assert.equal(r.status, 2, `stderr: ${r.stderr}`);
+    const expected = { reason: 'blocked-by-guard', from: 'hooks-standalone' };
+    assert.deepEqual(JSON.parse(r.stdout), expected);
+    assert.deepEqual(JSON.parse(r.stderr), expected);
+  });
+
+  // A5: sibling resolution. hooks/lib/cli-exit.js requires its OWN sibling
+  // (./exit-code-registry.js), not some other copy sitting elsewhere on the
+  // machine — proven by DELETING the sibling from the tmpdir and observing
+  // the require actually fail, then restoring it and observing success again.
+  test('resolves its OWN sibling exit-code-registry.js, not some other copy (A5)', (t) => {
+    const { dir, copiedExit, copiedRegistry } = makeStandaloneHooksCopy(t);
+    const registryBytes = fs.readFileSync(copiedRegistry);
+
+    // Sanity: with the sibling present, the copy loads clean.
+    const before = toLegacyResult(runNode(['-e', [
+      `const c = require(${JSON.stringify(copiedExit)});`,
+      `process.stdout.write(JSON.stringify({ hasTerminateNow: typeof c.terminateNow }));`,
+    ].join('\n')], { cwd: dir, timeoutMs: PROBE_TIMEOUT_MS }));
+    assert.equal(before.status, 0, `stderr: ${before.stderr}`);
+    assert.deepEqual(JSON.parse(before.stdout), { hasTerminateNow: 'function' });
+
+    // Prove the failure: delete the sibling, require must fail. Single-file
+    // removal of a fixture we immediately restore below (not directory
+    // teardown; cleanup(dir) still runs via t.after() for the whole tmpdir).
+    // eslint-disable-next-line local/no-raw-rmsync-in-tests -- see comment above
+    fs.rmSync(copiedRegistry);
+    try {
+      const missing = toLegacyResult(runNode(['-e', [
+        `require(${JSON.stringify(copiedExit)});`,
+      ].join('\n')], { cwd: dir, timeoutMs: PROBE_TIMEOUT_MS }));
+      assert.notEqual(missing.status, 0, 'require must fail once the sibling registry is removed');
+      assert.ok(
+        missing.stderr.includes('MODULE_NOT_FOUND') || missing.stderr.includes('Cannot find module'),
+        `expected a module-resolution failure naming the missing sibling; got: ${missing.stderr.slice(0, 400)}`,
+      );
+    } finally {
+      // Restore in a finally so a failing assertion above cannot leave the
+      // tmpdir fixture (owned by this test, not a committed artifact) broken
+      // for any later step in this same test.
+      fs.writeFileSync(copiedRegistry, registryBytes);
+    }
+
+    // Confirm restoration actually fixes it — the negative-space check is
+    // meaningless without this positive control.
+    const after = toLegacyResult(runNode(['-e', [
+      `const c = require(${JSON.stringify(copiedExit)});`,
+      `process.stdout.write(JSON.stringify({ hasTerminateNow: typeof c.terminateNow }));`,
+    ].join('\n')], { cwd: dir, timeoutMs: PROBE_TIMEOUT_MS }));
+    assert.equal(after.status, 0, `stderr: ${after.stderr}`);
+    assert.deepEqual(JSON.parse(after.stdout), { hasTerminateNow: 'function' });
+  });
+});
+
+// ─── #3911 A3: the --check generator guards can actually fail ──────────────
+//
+// CONTEXT.md's prove-it-can-fail rule: a guard that has never been observed
+// to fail is not a guard. For BOTH new committed artifacts, corrupt the
+// committed file, run the generator's --check, assert it fails and names the
+// file, then restore in a `finally` (so a failing assertion here can never
+// leave a committed artifact corrupted) and re-run --check to confirm the
+// restore actually cleared the guard.
+describe('#3911: the --check guards for the new hooks/lib artifacts can actually fail (A3)', () => {
+  const REPO_ROOT = path.resolve(__dirname, '..');
+  const GEN_HOOKS_CLI_EXIT = path.join(REPO_ROOT, 'scripts', 'gen-hooks-cli-exit.cjs');
+  const GEN_EXIT_CODE_REGISTRY = path.join(REPO_ROOT, 'scripts', 'gen-exit-code-registry.cjs');
+  // gen-hooks-cli-exit.cjs --check runs a real tsc compile of the whole
+  // project to a throwaway outDir (see its own COMPILE_TIMEOUT_MS=60000) —
+  // this needs a longer bound than a plain probe.
+  const CHECK_TIMEOUT_MS = 90000;
+
+  test('gen-hooks-cli-exit.cjs --check fails on a corrupted hooks/lib/cli-exit.js, names the file, and clears on restore', () => {
+    const original = fs.readFileSync(HOOKS_CLI_EXIT_PATH);
+    let corrupted = false;
+    try {
+      fs.appendFileSync(HOOKS_CLI_EXIT_PATH, '\n// corrupted-by-A3-test\n');
+      corrupted = true;
+      const r = toLegacyResult(runNode([GEN_HOOKS_CLI_EXIT, '--check'], { timeoutMs: CHECK_TIMEOUT_MS }));
+      assert.notEqual(r.status, 0, `--check must fail on a corrupted committed artifact; stderr: ${r.stderr}`);
+      assert.ok(
+        r.stderr.includes('cli-exit.js'),
+        `expected the failure to name the drifted file; got: ${r.stderr.slice(0, 400)}`,
+      );
+    } finally {
+      if (corrupted) fs.writeFileSync(HOOKS_CLI_EXIT_PATH, original);
+    }
+
+    const restored = toLegacyResult(runNode([GEN_HOOKS_CLI_EXIT, '--check'], { timeoutMs: CHECK_TIMEOUT_MS }));
+    assert.equal(restored.status, 0, `--check must pass again once the artifact is restored; stderr: ${restored.stderr}`);
+  });
+
+  test('gen-exit-code-registry.cjs --check fails on a corrupted hooks/lib/exit-code-registry.js, names the file, and clears on restore', () => {
+    const original = fs.readFileSync(HOOKS_EXIT_CODE_REGISTRY_PATH);
+    let corrupted = false;
+    try {
+      fs.appendFileSync(HOOKS_EXIT_CODE_REGISTRY_PATH, '\n// corrupted-by-A3-test\n');
+      corrupted = true;
+      const r = toLegacyResult(runNode([GEN_EXIT_CODE_REGISTRY, '--check'], { timeoutMs: PROBE_TIMEOUT_MS }));
+      assert.notEqual(r.status, 0, `--check must fail on a corrupted committed artifact; stderr: ${r.stderr}`);
+      assert.ok(
+        r.stderr.includes('exit-code-registry.js'),
+        `expected the failure to name the drifted file; got: ${r.stderr.slice(0, 400)}`,
+      );
+    } finally {
+      if (corrupted) fs.writeFileSync(HOOKS_EXIT_CODE_REGISTRY_PATH, original);
+    }
+
+    const restored = toLegacyResult(runNode([GEN_EXIT_CODE_REGISTRY, '--check'], { timeoutMs: PROBE_TIMEOUT_MS }));
+    assert.equal(restored.status, 0, `--check must pass again once the artifact is restored; stderr: ${restored.stderr}`);
+  });
+});